1. Field of the Invention
The field of the invention generally relates to power supplies and, more specifically, to a versatile DC output power supply.
2. Background of the Related Art
There are two main classes of power supply or converter: (1) AC to DC, and (2) DC to DC. An AC to DC power supply generally converts AC line voltage as its input to a DC output voltage and is found, for example, in applications such as home audio amplifiers. It can generally be implemented as either a linear or switching power supply. A DC to DC power supply converts from one existing DC voltage to another, for example from a battery, to another higher or lower voltage level. It is typically implemented with a switching power supply. For general use, DC to DC power supplies convert voltages and also provide isolation between input and output.
Common components of a conventional power supply include a transformer, rectifier, and smoothing/storage capacitors. Additional components commonly utilized in a switching power supply include a control IC chip, power transistors, filtering and screening to prevent electro-magnetic interference (EMI). The demand for ever smaller equipment has led to a preponderance of switching power supplies.
Conventional linear power supplies, used for instance in home audio amplifiers, use a large, heavy, expensive transformer to convert a low frequency, high-voltage AC line supply to a lower voltage suitable for the amplifier or other application. The high-voltage AC line supply is first dropped down to a lower AC voltage, and then the lower AC voltage waveform is rectified to DC. However, the rectified voltage is discontinuous and so large storage capacitors are needed in order to provide a smooth voltage for the amplifier. Even so, the DC supply still has an appreciable irregularity (the ripple voltage) superimposed upon the DC which can manifest as an audible hum and buzz at the amplifier output unless considerable care is taken with the amplifier design and layout.
While the design of such a power supply is relatively simple and the EMI emissions relatively low, the transformer is large, heavy and very expensive. The storage capacitors are also large and expensive. Thus the overall bulk of this power supply approach precludes its use on lightweight, low profile designs. The power losses in the power supply are relatively low, with an overall efficiency generally found in the 85-90% range.
An alternative to using linear power supplies is to employ a switched-mode power conversion technique. In this technique, the line voltage is first of all rectified and smoothed at full line voltage. This allows the storage capacitor to be smaller as compared to the linear power supply, and also less expensive. The resulting high voltage DC signal is then converted to a lower voltage by chopping it at a very high frequency—several tens of kHz typically—to produce an AC output signal which is transformed down to a lower voltage through a small transformer. Because the operating frequency is much higher than with a linear power supply, the transformer can be much smaller than in a conventional linear power supply. However, the AC signal on the output side of the transformer again has to be rectified to obtain DC and must still be smoothed with storage capacitors, albeit smaller ones than in a linear power supply. An example of such a power supply is an external power supply generally used to power a laptop computer.
One penalty to be paid in this approach is that, in order to retain efficiency, the chopping of the DC produces high frequency AC with a discontinuous, square waveshape. Such a waveshape generates high levels of very high frequencies which radiate to cause radio frequency interference (EMI). Careful design, layout and screening are required to reduce these emissions to an acceptable limit. The switching frequency components also need to be removed or isolated from the input and output lines, requiring extra magnetic components that add to the cost and bulk of the supply. The efficiency, although theoretically capable of being very high, typically lies in the 80-90% range. Overall, the size and weight of the switched-mode power supply can be reduced considerably compared to a conventional linear power supply and the basic component cost can also be lower. However, the complexities inherent in the design of a switching power supply can add considerably to the design and certification costs and result in a time to market of many months.
In sum, linear power supplies tend to be larger in size and profile, relatively costly, and heavy. They are advantageous in terms of efficiency and low EMI. Switching power supplies tend to be smaller and weigh less. Due to higher frequency operation, the transformers and capacitors of a switching power supply tend to be smaller than with a linear power supply. However, switching power supplies can be less efficient than linear power supplies, and produce significantly more EMI which requires careful filtering and screening. Switching power supplies are also more complex, needing control circuitry and power switching devices. They take longer to design and are generally more expensive than linear power supplies. The trend is towards ever smaller power supplies, requiring higher frequency operation and hence more potential issues relating to EMI.
Larger power supplies may utilize three-phase power generation, which is an alternative power supply technique to the ones thus far described. In a three-phase system, three power lines carry three alternating currents of the same frequency but different phases, which reach their instantaneous peak values at different times. The current waveforms are offset by 120 degrees from one another (that is, each current is offset by one-third of a cycle from the other two waveforms). This staggering of waveforms allows energy to be continuously provided to the load(s), with a reduced but nonetheless substantial ripple. As a result, a constant amount of power is transferred over each cycle of the current. Transformers may be used to step-up or step-down the voltage levels at various points in a three-phase power network. A three-phase rectifier bridge commonly includes six diodes, with two diodes used for each branch of the three-phases.
While three-phase power supply systems have some benefits, they are also subject to certain drawbacks or limitations. For example, a minimum of three conductors or power lines is generally required, as well as three sets of circuitry for level-shifting (with transformers) and rectifying each branch. Also, while ripple is reduced over a single-phase power supply, the ripple is still substantial and in general requires storage capacitors to bring down to an acceptable level.
A need exists for a power supply or converter that can be made small, lightweight and reasonably inexpensive, with minimal EMI. A need further exists for such a power supply that avoids the complexities and complications of a switching power supply. A further need exists for a power supply that can reduce the need for large components and thus be made small in size and profile and lightweight.